The present invention relates to coating of optical fibers in general, and more particularly to a method of an arrangement for coating optical fibers with metallic materials, especially immediately after the fiber is drawn from an optical preform.
In the manufacture of optical fibers, especially such to be used in the telecommunication field, it is often necessary to provide the optical fiber with a metallic coating, especially hermetic metallic coating, at its outer surface, if for no other reason, then to protect the fiber from deleterious environmental influences which could damage the mechanical and optical properties of the optical fiber.
Having recognized this need for providing metallic coating on the external surface of the optical fiber, there have been already proposed, and the optical fiber manufacturing industry uses, various methods of applying the desired metallic coatings to the optical fiber. Probably the most widely currently used method of coating glass fibers with metals, such as aluminum or indium, is by molten metal-dip coating. In such a coating method, the metal to be applied is molten in a coating cup and the fiber is passed through the molten metal accommodated in the coating cup. While most of the work involving molten metal-dipped coating was done with aluminum and indium, other metals, such as antimony, zinc and Al-Si were also tried.
However, experience has shown that several serious problems are associated with this process. Such problems include, among others, initial strength degradation due to fiber surface damage from abrasive oxides formed in the molten metal, development of pinholes in the applied metallic coating layer, and microbending losses due to the presence of a thick, non-uniform coating. Another disadvantage of this method is that it is limited to metals and alloys having relatively low melting temperatures, so that the optical fiber manufactured by using this method can not be used in relatively high temperature environments.